1. Field of the Invention
The present invention relates to a method and apparatus for playback of data recording disks, and in particular to a method and apparatus for playback of data recording disks (referred to hereinafter simply as disks) having digital signals recorded thereon.
2. Description of Background Information
Small-diameter audio disks having a diameter of approximately 12 cm and having digital signals recorded thereon, generally called "compact disks" or CDs, are now well known. However in recent years another type of disk, referred to in the following as a composite disk, has been developed, having an area in which is recorded an FM modulated video signal with a PCM signal superimposed thereon, as well as an area having only a PCM signal recorded. With such a composite disk, the area having only PCM data recorded, e.g. audio data in PCM form, may for example be disposed in an inner peripheral area of the disc, (this area being referred to hereinafter as the CD area), while the area containing the FM video signal with superimposed PCM signal (this area being referred to hereinafter as the video area) may be disposed peripherally outward from the CD area, separated therefrom by a predetermined radial distance.
A video signal contains higher frequency components than those of the PCM signal of the video area, so that the spectrum of the signals recorded in the video area will be as shown in FIG. 7, in which A denotes the PCM signal frequency components, and B denotes the video FM signal frequency components. At the time of recording the video area signals, the speed of rotation of the disc must be made higher than during that used during recording of the CD area, and therefore of course during playback it is necessary to rotate the disc at a higher speed during video area playback than during CD area playback. As shown in FIG. 8, the speed of disc rotation during CD area playback is several hundred r.p.m., whereas during video area playback the speed of rotation is two thousand plus several hundred r.p.m. for playback from the innermost periphery of that area, and is one thousand plus several hundred r.p.m. for playback from the outermost periphery of that area, so that the speed of rotation is extremely high during video area playback.
Thus, if it were attempted to use a prior art type of CD disc playback apparatus for reading recorded data from the video area of a composite disc, the frequency of the error signal supplied to the servo system which controls the position of the data detection point of the pickup with respect to the disc would be substantially higher than during CD data read-out. Since the servo loop gain is lowered when a high-frequency error signal is applied, there would be a loss of positioning control for the data detection point during such video area playback operation, so that it would be difficult to achieve satisfactory reading of video data from the disc.
In order to perform playback of a composite disc therefore, since the speed of rotation of the disc is extremely different between CD area and video area playback, it is necessary to change the disc rotation speed each time the playback area is changed. Furthermore in order to minimize costs, it is desirable to use a common demodulation system for the playback digital signals produced from both the video area and the CD area, and therefore the signal processing system for the playback digital signals must be capable of meeting this requirement. In addition, if such a playback apparatus is to be made capable of playback of both CD discs and composite discs, then various other requirements will be imposed on the apparatus.
Since with composite disc playback, as stated above, the disc speed of rotation is extremely different for CD area playback and video area playback respectively, it is necessary for the various servo systems of the apparatus to display different frequency characteristics for video area playback and CD area playback respectively, due to the higher frequency of the error signals which are supplied to the servo systems during video area playback. Furthermore, also as a result of the higher speed of rotation during video area playback, the overall servo loop gain will be low, due to the effects of various component elements in each loop such as actuators etc., which behave as intergrator elements. Measures to compensate for this loss of gain must also be provided.
Moreover, also due to the high speed of rotation during video area playback as compared with CD area playback with a composite disc, problems will arise with respect to track jumping. Specifically, the high speed of rotation during video area playback will magnify the adverse effects of any disc deviations such as eccentricity, and this must be taken into consideration if accurately determined amounts of track jumping are to be executed within the video area and the CD area.
Furthermore, various color television standards are used at present for the video data recorded in the video area, i.e. the NTSC and the PAL standards. If the television standard of the disc does not conform to that of the playback apparatus, then synchronization for video playback will not be possible, so that video data playback cannot be executed. However, in the case of a composite disc, video data are only recorded in one area of the disc, whereas audio data are recorded throughout the entire disc. Thus, it is undesirable that is should be completely impossible to perform playback of any data whatsoever from a composite disc, simply because the television standard of the video data is different from that of the playback apparatus.
Also, since the data recorded in the video area and CD area are respectively different, as are the playback speeds of rotation for each area, it is necessary to apply appropriate speed control for playback of each area, while in addition it is necessary that both video data and audio data playback can be performed simultaneously in the video area.